As a follow-on to the above comment - check and make sure the dealer installed the correct filter.
On edit. In fact, I think I would change out the filter and add a qt of oil and see if that cures the problem.
You had no problems until the last oil change. In a situation like this, I always look back at the last change that could have caused the problem. In this case I wonder if they installed the wrong filter or if maybe the check valve in the filter is somehow malfunctioning and periodically stopping the flow of oil out of the filter. I know this isn't supposed to happen but . . . . . . .
I used to have a Duramax and remember how high the oil pressure ran. If it's not the oil pressure switch, then it seems like something is drastically affecting the flow of oil. In my opinion, the first item on the list would be the oil filter, especially since you recently had it changed.
I would think even the wrong wt oil would not cause a problem like you are experiencing.